A floating ship has six degrees of freedom, roll, pitch, yaw, heave, surge, and sway. Roll is generally the most objectionable as it is easily magnified by sea conditions, it affects sea keeping, operation of the ship, and the ship's course, and can damage cargo being stored on the ship. It is also unpleasant for passengers and the crew by causing motion induced sickness. All vessels have their own natural roll period depending on hull shape, loading, and other factors. Wave motions initiate this roll and, if the wave encounter frequency is in close synchronization with the vessel's natural period, roll motion may build to uncomfortable or even dangerous proportions. A vessel will naturally exhibit wave-induced roll both while making headway (“underway”) and while drifting, holding position or on anchor at zero forward speed (“at rest”).
Many types of stabilizing systems have been developed to dampen wave-induced roll motion. The most prevalent type of stabilizing system involves the use actively-controlled underwater fins to generate the forces used to stabilize a vessel making headway. When used underway, fins are rotated about the shaft stock axis presenting an angle to the onrushing water which generates a hydrodynamic lift force.
More recently, active underwater fin stabilizer systems have also been utilized to dampen vessel roll motion while the vessel is at rest (zero forward speed). When used at rest, fins are rotated about the shaft stock axis and act on the surrounding water in such a way, not unlike a paddle, to create a useful force. Fin systems that are designed to operate at zero forward speed are commonly referred to as “stabilization at anchor”, “at rest”, or “zero speed” systems. Because these stabilizer systems attempt to satisfy the vessel's roll reduction requirements both while underway and at rest, a design compromise exists. A fin planform geometry optimized to suit one requirement (e.g. at rest) will not be well-configured for the other requirement (e.g. underway). Moreover, the fin area required to stabilize a vessel at rest is typically larger (often significantly larger) than the fin area required to stabilize the same vessel underway. The smaller area required for underway stabilization is due to the hydrodynamic benefits which stem from the fin's movement during forward motion through the water. Consequently, a large fin area sized and shaped for at rest stabilization causes significant inefficiencies, including higher total drag and a poor lift-to-drag ratio, when the same shape is also used to satisfy underway stabilization requirements.
Prior art systems, such as U.S. Pat. No. 7,451,715 to Koop et al., have attempted to overcome this problem by introducing a fin stabilization system where the fin has an extension portion that extends the body of the fin; the extension is deployed from inside of the fin itself. However, this fin design suffers from at least one major problem. Since the extension is perfectly flat, there is minimal drag, and it is inefficient at trapping water. This creates an inefficient mechanism of controlling the roll of the ship at rest as the water can readily pass over the extension.
What is desired, therefore, is a variable geometry fin designed to more efficiently adjust the flow of water and readily create a greater amount of drag for a given rotational speed and area, and which facilitates ship stabilization at rest or at slow speeds underway more efficiently, while also allowing for efficient stabilization underway at higher speeds.